The "iLab" visual field diagnostic method
The instrument consists of a head mounted eye tracking system (T) communicating wirelessly with a computer (C.) and a test screen (S) which is a conventional flat screen monitor with enhanced viewing angle. Because the stimulus and fixation target can be anywhere on the screen, the effective field of view covers up to 100 degrees horizontal. Accuracy and repeatability are better than a degree on detail plots (as described below) and repeatability of the standard test at a resolution of 5 degrees is close to absolute. The iLab test abolishes the need for head restraint and concentrated staring at a single fixed point. This allows the patient to behave naturally and greatly improves involuntary performance. In essence a series of points are presented such that the patient (P) naturally saccades from one to the other.
Research has shown that attention is held better if a fixation target is more complex than a point and has animated edges. So the iLab primary fixation target is a ladybird (L) with wiggly legs. The ladybird wanders randomly across the test screen while the computer selects test points which are required to complete the visual field plot and which fit on the screen depending on the position of the ladybird and on the patient's head position as measured by an ultrasonic (U) sensor. The patient may thus move their head naturally and is encouraged to do so as it is part of the desired natural behaviour. It also reduces the eyelid droop artefact. The test points are presented and the precise time a saccade is triggered indicates the integrated quantity of light falling on the retina at that moment. This measurement takes into account the retinal integration time and the delay caused by the Pullfrich effect (the well known stereo optical illusion produced with a pendulum and monocular neutral filter) to calculate an approximation to the actual sensitivity of the retina at that point. The accuracy of the measurement of retinal sensitivity is typically 2 dB and can be as high as 1 dB per presentation. Once a defect has been detected a detailed plot of the affected area can be made and the data is enhanced further to improve the accuracy of each point. The practitioner (PR) can "mouse click" an area of interest to interactively alter the region being tested.
In summary the iLab presents a sequence of test points to the patient who simply "watches" the screen while the instrument detects and times the saccades from point to point. The stimulus point changes from a plain point to the ladybird at the moment the patient looks at it, and the original ladybird disappears.
The new test is by its very nature rapid, so a 64 point test can be accomplished in just over a minute including re-presentations for detected errors. In general, approximately 3 percent of test points will remain with errors but this compares favourably to conventional VFA methods and repeated "staircase" or "Swedish" threshold protocols. Even the short time taken may be inconvenient to be applied routinely so the instrument also provides an ultra fast initial screening test which makes use of its saccade timing methods. The patient is presented with a sequence of visual scenes designed to cause a differential in saccade rhythm between patients with defects and those with no defects. This test takes only a few seconds and so can be conducted easily on every patient as a matter of course.
The volunteer subjects have universally commented that the test is comfortable and even enjoyable. Thus we believe that this instrument and method have great promise for both diagnosis and management of treatment.
